Nonvertebrate Chordates, Fishes, and Amphibians Biology I: Chapter 30

CHORDATES

Chordates Phylum Chordata Fishes, amphibians, reptiles, birds and mammals Four key characteristics: 1. Dorsal, hollow nerve cord 2. A notochord 3. Pharyngeal pouches 4. Tail that extends beyond the anus

Dorsal, Hollow Nerve Cord Nerves branch from this cord at regular intervals Nerves connect to internal organs, muscles and sense organs

Notochord Long support rod Runs through the body just below the nerve cord Most chordates only have in the embryonic stage

Pharyngeal Pouches Paired structures in the throat (pharynx) region Fishes and amphibians: slits develop that connect the pharyngeal pouches to the outside of the body The slits may then develop into gills that are used for gas exchange

Tail All chordates have a tail that extends beyond the anus at some point in their lives The tail can contain bone and muscles and is used in swimming by many aquatic species

Most Chordates are Vertebrates More than 99% of all chordates are in the subphylum Vertebrata Vertebrae: individual segments that make up the backbone; encloses and protects the spinal cord Backbone is part of an endoskeleton, or internal skeleton

Nonvertebrate Chordates The two groups Soft-bodied marine organisms Have the four key characteristics at some point in their lives Tunicates Filter feeders Lancelets Small, fish-like creatures

Fishes Aquatic vertebrates that are characterized by: Paired fins Used for movement Scales Used for protection Gills Used for exchanging gases

Evolution of Fishes Fishes were the first vertebrates to evolve The evolution of jaws and the evolution of paired fins were important developments during the rise of fishes

The First Fishes Jawless creatures whose bodies where armored with bony plates Lived in the oceans during the late Cambrian Period, about 510 mya Fishes kept this armored, jawless body plan for 100 million years

The Age of Fishes Ordovician and Silurian Periods: 505-410 mya, fishes underwent a major adaptive radiation Devonian Period: “Age of Fishes” Some were jawless with little armor Ancestors of modern hagfishes and lampreys Others were armored and ultimately became extinct about 360 mya

The Arrival of Jaws Other ancient fishes kept their bony armor and possessed a feeding adaptation that would revolutionize vertebrate evolution: JAWS Jawless fishes Limited to eating small particles of food that they filter out of the water or suck up like a vacuum cleaner Jaws can hold teeth and muscles Much wider variety of food Defend themselves by biting

The Arrival of Paired Fins More control of body movement Fin tails and powerful muscles gave greater thrust when swimming Enabled fishes to move in new and varied patterns This enabled fishes to use their jaws in complex ways

The Rise of Modern Fishes Although the early jawed fishes soon disappeared, they left behind two major groups that continued to evolve and still survive today Ancestors of modern sharks and rays: skeletons made of resilient cartilage Group that evolved skeletons made of true bone

Form and Function in Fishes Adaptations to aquatic life include: Various modes of feeding Specialized structures for gas exchange Paired fins for locomotion

Feeding Herbivores, carnivores, parasites, filter feeders, detritus feeders A single fish may exhibit several modes of feeding (carp: eat what is available) while others are very specialized (barracuda: carnivore) Pyloric ceca: finger-like pouches found in many species of fish that secrete digestive enzymes to help digest food

Respiration Most fishes exchange gases using gills located on either side of the pharynx The gills are made up of feathery filaments Lampreys and sharks have several gill openings on the side of the body A number of fishes, such as the lungfish, have an adaptation that allows them to survive in oxygen-poor water or in areas where bodies of water often dry up

Respiration

Circulation Closed circulatory systems with a heart that pumps blood around the body in a single loop Heart consists of 4 parts: Sinus venosus: thin-walled sac that collects blood from the fish’s veins Atrium: a large muscular chamber that serves as a one-way compartment for blood Ventricle: thick-walled muscular chamber; pumping portion of the heart Bulbus arteriosus: connects to a large blood vessel called the aorta, which moves blood to the fish’s gills

Circulation

Excretion Most fishes rid themselves of nitrogenous wastes in the form of ammonia Gills, kidneys Saltwater fishes Lose water by osmosis: kidneys return as much water to the body as possible Freshwater fishes Gain water by osmosis: kidneys pump out plenty of dilute urine

Response Well-developed nervous systems organized around a brain Cerebrum: area of the brain responsible for all voluntary activities of the body Cerebellum: region of the brain that coordinates body movements Medulla oblongata: area of the brain that controls the functioning of many internal organs

Response Lateral line system: sensitive receptor system that enables fish to detect gentle currents and vibrations in the water (the 6th sense)

Movement Most move by alternately contracting paired sets of muscles on either side of the backbone Because their body tissues are more dense than the water they swim in, sinking is an issue for fishes Swim bladder: gas-filled organ found in many bony fishes that adjusts their buoyancy

Reproduction Oviparous: term used to refer to animals whose eggs hatch outside the mother’s body Ovoviviparous: term used to refer to animals whose young are born alive after developing in eggs inside the mother’s body Viviparous: term used to refer to animals that bear live young that are nourished directly by the mother’s body as they develop

Groups of Fishes Over 24,000 living species that are extremely diverse Jawless fishes Cartilaginous fishes Bony fishes

Jawless Fishes Have no true teeth or jaws Skeletons are made of fibers and cartilage Lack vertebrae; keep notochords as adults Two main classes: Lamprey Hagfishes

Sharks and Their Relatives Class Chondrichthyes Sharks, rays, skates, Also: sawfishes and chimaeras Cartilage, not bone

350 Living Species Curved tails Torpedo-shaped bodies Pointed snouts Mouth underneath Enormous number of teeth Always exceptions!

Bony Fishes Class Osteichthyes Skeletons made of hard, calcified tissue: bone Ray-finned fishes Rays or spines that support the fins Only 7 living species of bony fish are not ray-finned Lobe-finned fishes

Ecology of Fishes Anadromous: fishes that spend most of their lives in the ocean but migrate to fresh water to breed Examples: lampreys, sturgeons, and salmon Catadromous: fishes that spend most of their lives in fresh water but migrate to the ocean to breed Example: European eel, American eel

AMPHIBIANS

Amphibian Have survived for hundreds of millions of years The only modern descendants of an ancient group that gave rise to all other land vertebrates Amphibian means “double life”…live in both water and on land

Amphibian Vertebrate Lives in the water as a larva and on land as an adult (with some exceptions) Breathes with lungs as an adult Has moist skin that contains mucus glands Lacks scales and claws

Evolution of Amphibians The first amphibians to climb onto land probably resembled lobe-finned fishes similar to the modern coelacanth The amphibian had legs, appearing about 360 mya

Evolution of Amphibians Early amphibians evolved several adaptations that helped them live at least part of their lives out of water: Bones in the limbs and limb girdles became stronger for more efficient movement Lungs and breathing tubes enabled them to breathe air The sternum formed a bony shield to support and protect internal organs, esp. the lungs

Evolution of Amphibians Soon after they appeared, amphibians underwent a major adaptive radiation Some were huge: Eogyrinus was about 5 meters long! Amphibians became the dominant form of animal life in the warm, swampy fern forests about 360-286 mya Climate changes caused many of the swamps to disappear Most amphibians became extinct

Evolution of Amphibians Only three orders of small amphibians survive today: Frogs and toads Salamanders Caecilians

Form and Function in Amphibians Although the class Amphibia is relatively small, it is diverse enough to make it difficult to identify a typical species We will focus on the structures found in frogs…

Feeding Tadpoles Adult amphibians Filter feeders or herbivores that graze on algae Eat almost constantly Adult amphibians Almost entirely carnivorous Mouth esophagus  stomach  small intestines  large intestines  cloaca

Respiration Larval amphibians Adult amphibians Gas is exchanged through the skin and gills Adult amphibians Lungs and skin

Circulation Double loop 3 chambered heart First Loop Carries oxygen-poor blood: heart  lungs and skin Takes oxygen-rich blood: lungs and skin  heart

Circulation Second Loop Transports oxygen-rich blood: heart  rest of the body Transports oxygen- poor blood: body  heart

Circulation

Excretion Amphibians have kidneys that filter wastes from the blood Urine: The excretory product of the kidneys Urine travels through tubes called ureters into the cloaca Urine is passes outside or temporarily stored in a bladder above the cloaca

Reproduction Eggs do not have shells and tend to dry out if they are not kept moist Most species lay eggs in the water The male fertilizes them externally In a few species (most salamander), eggs are fertilized internally

Reproduction The male climbs onto the female’s back The female releases the eggs that are then fertilized The eggs are in a transparent jelly, useful for attaching the eggs to underwater plants and that provides nourishment to the growing cells

Reproduction Some amphibians abandon their eggs after they are laid Others incubate the young in their mouth, on their back, or even in their stomach!

Movement Amphibian larvae Often move very much like fishes, wiggling their bodies and using a flattened tail for propulsion

Movement Adult amphibians Most use their front and back legs to move in a variety of ways Jump, climb, run, etc.

Response Nictitating Membrane: Moveable transparent membrane in amphibians located inside the regular eyelid Protects the surface of the eye from damage under water and keeps it moist on land

Response Tympanic Membrane Eardrum of amphibians Inside the skull Vibrates in response to sound, allowing hearing

Groups of Amphibians Salamanders: Order Urodela Long bodies and tails Four legs Carnivores Frogs and Toads: Order Anura Frogs: Long legs and lengthy jumpers Toads: short legs and short hops Adults lack tails Caecilians: Order Apoda Legless animals that live in water or burrow in moist soil or sediment

Ecology of Amphibians Have no feathers, fur or scales Many are ideal for predators such as birds and mammals

Ecology of Amphibians Amphibians have adaptations to protect themselves Many have skin colors and markers that allow them to blend in with the environment Some release toxins Some are brightly colored as a warning to predators